There are many types of rock bolt devices commercially available for installation within bore holes that are drilled into earthen formations. Rock bolts provide reinforcement in various settings, such as underground mining sites throughout the world. Rock bolts may also be referred to as rock stabilizers and ground stabilizers.
Rock bolts can be grouped into classes including, for example, friction rock bolts, expansion rock bolts, and cement/resin rock bolts. Rock bolts are typically inserted into a drilled bore in an earthen formation such as a rock formation. The drilled bore is most often formed in a ceiling surface of an underground tunnel. The rock bolt is held in the bore with a friction or adhesive interface with an inner surface of the bore. The rock bolt may be used to attach a wire mesh or other retention structure against the ceiling of the underground tunnel. The type of rock bolt and the interaction between the rock bolt and the drilled bore determine an amount of pull strength or load that the rock bolt provides. Friction rock bolts usually provide the lowest amount of pull strength, but are typically easier to install and are the least expensive rock bolt option. Expansion rock bolts and cement/resin rock bolts usually provide greater pull strength, but are often more expensive and complex to install.
Friction rock bolts generally comprise an elongate tube of a substantially circular cross-section and a channel or groove extending longitudinally along the entire length of the tube. Friction rock bolts are usually installed in a bore that has a smaller diameter than the outer diameter of the friction rock bolt. The friction rock bolt is driven into the hole and held in place within the bore with an interference fit. The tube is subject to radial compression forces as a result of being driven into the bore, which causes the channel or groove to be reduced thereby reducing the diameter of the tube to conform to the diameter of the bore. The resulting frictional engagement between the friction rock bolt and the earthen formation is sufficient to provide a load carrying capacity (e.g., pull strength) for the friction rock bolt.
Expansion rock bolts usually have a smaller outer diameter than the diameter of the bore into which the rock bolt is inserted. A radially outward directed force is applied internally within the rock bolt to expand the rock bolt radially outward to create a friction interface with the bore inner surface. In one example, the expanding mechanism is a wedge that is forced internally within the tube, and the wedge creates the radially outward directed force that expands the diameter of the rock bolt into contact with the bore inner surface. In another example, a fluid such as water is forced into the interior of the tube thereby creating a radially outward expansion force. The applied radial expansion force alters a shape of the tube, thereby causing permanent deformation that maintains a frictional interface with the bore.
Cement/resin rock bolts use injection of a cement or resin composition between an outer surface of the inserted tube and the inner bore surface to create a connection therebetween. The cement/resin is allowed to cure thereby providing a bond between the outer surface of the tube and the bore surface. This bond provides the desired frictional interface between the rock bolt and the bore.
The processes involved in handling fluids used in expansion rock bolts as well as the cements and resins used with cement/resin rock bolts may add significant time, cost, and complexity to installation of such rock bolts, which may, in at least some circumstances, outweigh their potential added benefits of increased load-carrying capacity.
Therefore, there is a need for improvements in rock bolt designs and related methods of installing rock bolts.